1. Field of the Invention
The present invention pertains to gamma ray imaging devices and more particularly to high-resolution, gamma ray imaging device for scintimammography and other medical diagnostic applications.
2. Related Prior Art
Gamma ray imaging devices have reached a point in their development that a general multi-purpose imaging device is not feasible. Presently, problems relate to the generation of gamma rays and their detection.
Application specific detectors useful for scintimammography must have a suitable physical geometry for breast imaging, be able to efficiently detect and locate gamma rays emitted from lesions in breast tissue, and at the same time efficiently reject scattered radiation from the patient body. These gamma ray imagers could be used as additional diagnostic tools used in conjunction with X-ray mammography. The following articles generally are representative of the state of the art regarding scintimammography and apparatus used in breast imaging.
An article titled "Scintillating Array Gamma Camera for Clinical Use", written by R. Pani et al. relates to dedicated gamma cameras for specific clinical application. These cameras are based on the use of position sensitive photo multiplier tubes (PSPMT) . The main intrinsic limitation of large area position sensitive photomultiplier tube (5" diameter) is the photocathode glass window. Coupling planar scintillation crystal strongly affects the useful active area and the intrinsic spatial resolution. To overcome this limitation the first 5" diameter gamma camera was developed that consisted of Hamamatsu R3292 position sensitive photomultiplier tube coupled to 50.times.50 YAP: Ce scintillating array. The array pixel size was 2.times.2 mm.sup.2 and the overall dimension of multi-crystal was 10.times.10.times.1 cm.sup.3. Resistive chains to calculate the centroid were used. A scintillating array produces a focused light spot minimizing the spread introduced by position sensitive photomultiplier tube glass window. The intrinsic spatial resolution resulted between 2 mm and 2.7 mm. The position linearity and useful active area resulted in good agreement with the intrinsic one obtained by light spot irradiation. The real limitation resulted the poor energy resolution of crystals (forty percent) and the poor uniformity response of position sensitive photomultiplier tube (within +or -fifteen percent). A correction matrix was then carried out by which fifty-seven percent of total energy resolution was obtained. The camera operates as Single Photon Emission Mammography (SPEM) and is producing breast functional images for malignant tumor detection under breast compression and in the same geometry of standard x-ray mammography.
An article titled "Multi-crystal YAP: Ce Detector System for Position Sensitive Measurements", written by R. Pani et al in Nuclear Instruments and Methods in Physics Research, A 348 (1994) relates to Yttrium aluminum perovskite (YAP:Ce) scintillation crystal. YAP:Ce has a light efficiency of about forty percent relative to NaI. Because of the yttrium atomic number (Z=39) and the relatively high density (5.37 g/cm.sup.3) this crystal has a good gamma-ray absorption. Furthermore it is not hygroscopic and is inert. Its peculiarity consists of material processing that provides us with crystal pillars down to 0.3.times.0.3 mm.sup.2 aperture size and up to some centimeters in length. An array consisting of 11.times.22 YAP:Ce elements was made where each crystal has an aperture of 0.6.times.0.6 mm.sup.2 and a length of 7 mm. Each scintillation crystal is optically separated by a reflective material resulting in a separation layer between elements of about 5 .mu.m. The multicrystal detector is optically coupled to a Hamamatsu Position Sensitive Photomultiplier Tube (R2486). The intrinsic spatial resolution of the position sensitive photomultiplier tube is better than 0.3 mm but it is strongly dependent on the Point Spread Function (PSF) generated on the photocathode. The multicrystal detector very well matched the position sensitive photomultiplier tube characteristics resulting in a spatial resolution of about 0.7 mm at 140 keV (.sup.99m Tc) gamma irradiation.
An article titled "Toward a Nuclear Medicine with Submillimeter Spatial Resolution", written by L. H. Barone et al in Nuclear Instruments and Methods in Physics Research, A 360 (1995) relates to the HIRESPET Collaboration developing a new concept of a gamma camera with sub-millimeter spatial resolution. The first prototype consists of a small field size gamma camera based on a position sensitive photomultiplier tube (PSPMT) coupled to a scintillation crystal. The intrinsic spatial resolution of the position sensitive photomultiplier tube is better than 0.3 mm. The scintillation crystal consists of yttrium aluminum perovskite (YAP: Ce). It has a light efficiency of about forty percent relative to NaI, a good gamma radiation absorption (z=39) and a high density (5.37 g/cm.sup.3). It is inert and not hygroscopic. To match PSPMY characteristics, a special crystal assembly has been made consisting of a bundle of yttrium aluminum perovskite pillars, where a single crystal has the transversal dimension of 0.6.times.0.6 mm.sup.2 and a thickness ranging between 1 mm and 28 mm. Each scintillation pillar is optically separated from the other by a reflective layer of 5 .mu.m thick. The preliminary results obtained from the gamma camera prototype (yttrium aluminum perovskite camera) show spatial resolution values ranging between 0.6 and 1 mm and an intrinsic detection efficiency comparable with a standard Anger camera.
An article titled "Pixellated CsI(TI) arrays with position-sensitive PMT readout", written by A. Truman et al in Nuclear Instruments and Methods in Physical Research, A 353 (1994), relates to the position and energy resolution characteristics of three scintillation detectors viewed by a three square inch position sensitive photomultiplier tube that have been measured as a function of photon energy. Pixellated detectors having a pitch that ranges between 1.5 mm and 3.5 mm have been studied. The FWHM of the distribution in measured positions was as little as 0.9 mm at 122 keV. In this case, the tube was read out using individual amplifiers to record the charge detected on each individual anode wire and the location found using a peak fitting algorithm. Comparative measurements were also made using the conventional hardware centroiding technique.